Powder metal parts are currently being used in a wide variety of high volume applications due to their substantially lower costs. However, because of current strength limitations of powder metal components, applications in vehicle power transmissions have been limited only to lower-loaded components. Although powder metal gears are increasingly being used in powered hand tools, gear pumps, and as accessory components in automotive transmissions, powder metal gears have not been used for power transmission gearing. The state-of-the-art powder metal gears do not possess adequate tooth bending strength and pitting/wear resistance as compared to gears produced from wrought and/or forged steels.
Highly loaded gears used for power transmission gearing are conventionally manufactured from wrought and/or forged low carbon, low to medium alloyed steel blanks. After preliminary blank machining, gear teeth are produced by metal cutting operations such as hobbing or shaping, or by forging to near net shape. Gears are then heat treated to impart desired surface strength, strength gradient and core toughness. Heat treatment involves carburizing the surface of low carbon steel gears to increase the surface and near surface carbon content, followed by hardening by rapid quenching to below the temperature (Ms) at which a diffusionless transformation process that creates a hardened martensitic structure proceeds to completion. Alternatively, gear wheels produced from medium to high carbon alloy steel compositions, which therefore do not need carburization, are instead induction hardened, wherein only the gear tooth surfaces are heated and then quenched to produce the hardened martensitic structure. The hardened gears are then finished to net shape by grinding, skiving, burnishing, and/or honing operations.
For powder metal gears, a method has been described in U.S. Pat. No. 5,711,187, wherein a powder metal gear wheel formed from a pressed and sintered powder metal blank is claimed to be surface hardened by densifying the tooth surface layers, both in the flank and root/fillet region. This patent describes a pre-finishing technique of gear rolling that is performed prior to heat treatment and hardening using either a single-die or double-die rolling apparatus, and is applicable for sintered low alloy steel compositions similar to SAE 4100, SAE 4600, and SAE 8600 grades. However, as the method described in U.S. Pat. No. 5,711,187 is a pre-finishing operation that is performed prior to heat treatment and hardening, it is applicable only to low carbon low alloy sintered steel compositions in the soft machinable condition, particularly compositions with carbon contents of 0.2% or less. This patent claims full theoretical densification at the rolled surfaces and a progressively decreasing densification (90-100%) gradient of at least 380 microns up to about 1000 microns in depth.
As noted above, the method described in U.S. Pat. No. 5,711,187 is applicable only to relatively soft gear wheel blanks made of low carbon low alloy sintered powder metal steel compositions with hardness typically less than BHN 180 (or HRC less than 24). Gear rolling of soft sintered gear tooth surfaces as described in U.S. Pat. No. 5,711,187 produces densification of tooth surface layers. As has been noted above, powder metal gears, either in the as-sintered condition or after surface densification by gear rolling as described in this patent, have to be heat treated by carburizing and hardening operations to achieve the specific surface hardness, hardness gradient and core strength necessary for high load bearing power transmission gearing. Any surface hardening achieved due to work hardening by gear rolling and related surface densification as described in U.S. Pat. No. 5,711,187 is substantially eliminated during the subsequent heat treatment process.
Furthermore, because the sintered and densified powder metal gears produced by the method described in U.S. Pat. No. 5,711,187 are subjected to heat treatment and hardening, the gears may require subsequent hard finishing by grinding, skiving, burnishing or honing operations to achieve the required level of accuracy, resulting in removal of about 150 microns of the densified surface region of gear teeth. This removal of the portion of the surface region with improved apparent hardness of powder metal densified surface layers lowers the load bearing capacity.
Apparatus and methods have been described in U.S. Pat. No. 5,221,513; U.S. Pat. No. 5,391,862; U.S. Pat. No. 5,451,275; U.S. Pat. No. 5,656,106; U.S. Pat. No. 5,799,398; U.S. Pat. No. 6,007,762; and U.S. Pat. No. 6,126,892 for wrought and/or forged steel gear wheels and U.S. Pat. No. 6,264,768 for rolling element bearings in which a carburized and hardened workpiece is finished by thermomechanical means by inducing controlled plastic deformation in the metastable austenitic condition via gear rolling. Such a thermomechanical treatment, also called ausform finishing, of hardened gear tooth surfaces involves reaustenitization by induction heating followed by marquenching at about 450° F. to 500° F. or above the start of the martensite transformation temperature (Ms). The gear teeth in this marquenched condition are roll finished and then finally quenched to martensite before any diffusional decomposition can form from the metastable austenite. For wrought and/or forged gear wheels, the thermomechanical method of ausform finishing described in the above-identified patents results in substantial material flow up and down the tooth surfaces and in the axial direction due to combined rolling and sliding action on the tooth surfaces. Unlike conventional gear finishing such as grinding, the outermost surface hardened layers are not removed during the ausform finishing operation.
The method described in the previously mentioned patents is also applicable to medium to high carbon alloyed gear steels, wherein the carbon content is sufficiently high such that the carburizing operation is not required. The thermomechanical procedure described in the patents is thus applicable to both low carbon carburized/hardened gear steels as well as medium to high carbon induction hardenable gear steels and is employed in the present invention.
United States Patent Application Publication No. 2004/0219051 discloses a technique wherein sintered and hardened gear wheels are surface densified, hardened, strengthened, and finished to high accuracy by thermomechanical means in the metastable austenitic condition. This simultaneously occurs in the gear tooth flanks and in the root/fillet regions by substantial surface compaction during the rolling operation.
In accordance with United States Patent Application Publication No. 2004/0219051 there is provided a method and apparatus for densification by surface compaction and roll finishing of sintered and hardened powder metal gear wheels by thermomechanical means in the metastable austenitic condition, both on the flanks and in the root/fillet regions of gear teeth, resulting in surface densification to fully dense at the surface and 95-100% in the near surface region. This produces enhanced apparent surface and near surface hardness, improves mechanical properties due to ausforming, and produces a dimensional accuracy and surface finish comparable to or better than hard grinding, thereby eliminating the need for any subsequent finishing operations. The method described by United States Patent Application Publication No. 2004/0219051 is reported to be applicable to both sintered low carbon alloy powder metal gear steels that are carburized and hardened prior to the thermomechanical finishing treatment, and to sintered medium to high carbon alloy powder metal gear steels that are induction hardenable.
The method described by United States Patent Application Publication No. 2004/0219051 for sintered and hardened powder metal gears is reported to result in surface densification to 100% theoretical density at the surface, with progressively reducing densification of 95 to 100% produced at least in the outer 400 microns, and possibly up to 1300 microns. Furthermore, as the procedure of United States Patent Application Publication No. 2004/0219051 is the final finishing operation for hardened powder metal gears, the full benefit of the surface densification achieved and the related enhanced apparent surface hardness, finished gear strength, accuracy and surface finish, are fully retained. Finally, the plastic deformation induced in the metastable austenitic condition by thermomechanical means induces additional strength due to ausforming effects, thus resulting in further enhanced strength of the gear wheels.
As the powder metal sintered and heat treated gear wheels contain substantial amounts of pores prior to densification with effective density in the range of 90-95% of theoretically fully dense alloy, the rolling dies used for thermomechanical finishing are required to be designed specifically for densification by rolling involving substantial compaction of the material in the tooth surface layers. In contrast, the rolling dies for thermomechanical finishing of gear wheels made of wrought or forged steels are designed for combined rolling and sliding action on the tooth surface layers. The material flow is lateral oriented both in the tangential direction up and down the gear teeth as well as in the axial direction as no radial compaction of the material is possible. Therefore, for the thermomechanical finishing of powder metal sintered and heat treated gear wheels, the rolling dies apply surface densification pressure resulting in a collapse of the pores near the tooth surface region. This results in densification. The shapes of the rolling die, especially the die tooth tips, are designed for conjugacy, for contacting the gear wheel in the regions of interest and for compressing the material.
In order to achieve a nominally involute rolled gear wheel tooth profile in the finished condition for sintered and hardened powder metal steel gears, the rolling dies' tooth profile must substantially deviate from nominal involute tooth geometry. As the method of United States Patent Application Publication No. 2004/0219051 involves induction heating of the gear tooth surfaces followed by marquenching to temperatures in the range of 450° F. to 500° F. and then plastic deformation and compaction of surface layers by gear rolling, the rolling dies are maintained at the processing temperature of 450° F. to 500° F. and therefore are subjected to substantial thermal expansion. Due to the rolling die thermal expansion, the die tooth profile at the elevated operating temperature is substantially different from the initial rolling die tooth profile at room temperature and as originally produced. Similarly, the gear is not only roll finished at the elevated temperature of 450° F. to 500° F., but is also subjected to localized heating of the surface layers by induction heating followed by marquenching.
United States Patent Application Publication No. 2004/0219051 describes a procedure wherein the gear is thus subjected to a complex thermal history as well as associated metallurgical transformations. The resulting volumetric dimensional changes in the gear tooth profiles thus result in substantial deviation from the initial gear tooth profiles at room temperature and as originally produced. The gear teeth in the thermally and metallurgically modified geometrical shape and state are then rolled against the thermally modified rolling dies under high loads and speeds. The gear teeth are thus subjected to plastic deformation and densification at the elevated temperature by rolling pressure applied by the thermally modified rolling dies.
The roll finished and densified gear, still at the elevated temperature, is then finally quenched to room temperature and/or further below the Mf temperature. It is in the finally quenched condition that the sintered, hardened and rolled/densified gear wheel is in conformance with the specified nominally involute gear geometry condition. Predicting and implementing the required initial specialized rolling die tooth profile is critical for achieving the desired contact along the gear wheel tooth surfaces and the desired degree of compaction and densification in the flank and root/fillet regions of the gear teeth.
In order to produce wrought or forged steel gears with improved accuracy, surface finish and enhanced load carrying capacity, United States Patent Application Publication No. 2004/0219051 notes that the gear roll finishing process must be applied to both the active contacting surfaces as well as the trochoidal root fillet regions of the helical gear teeth. The apparatus and methods to this end have been disclosed in U.S. patent application Ser. No. 10/056,928 of Nagesh Sonti et al. As therein explained, if the roll finishing operation were extended to finish the root/fillet regions in addition to the active contacting surfaces of the gear teeth, then the surface finish and bending fatigue strength of the gear teeth would be substantially improved. Root fillet regions of gear teeth experience the maximum bending stress. Roll finishing of the root/fillet regions improves the surface finish, thereby reducing the stress concentration, and enhances the fatigue resistance of the material due to plastic working.
United Stated Patent Application Publication No. 2004/0219051 A1 more specifically discloses a method for net shaping gear teeth of a high performance power transmission gear from a powder metal workpiece, comprising the steps of: (a) heating a powder metal workpiece in the form of a near net shaped gear blank having gear teeth surfaces above its critical temperature to obtain an austenitic structure throughout its surfaces; (b) isothermally quenching the workpiece at a rate greater than the critical cooling rate of its surfaces to a uniform metastable austenitic temperature just above the martensitic transformation temperature; (c) rolling the gear teeth surfaces of the workpiece to a desired outer peripheral profiled shape between opposed dies, each die having an outer peripheral profiled surface, while holding the workpiece at the uniform metastable austenitic temperature, the gear teeth surfaces undergoing densification, plastic deformation, and strengthening as a result of the rolling operation; and (d) cooling the workpiece through the martensitic range to thereby harden the surfaces of the gear teeth.
United Stated Patent Application Publication No. 2004/0219051 A1 further discloses an apparatus for net shaping gear teeth of a high performance power transmission gear from a powder metal workpiece comprising: a source of heat for heating a powder metal workpiece in the form of a near net shaped gear blank having carburized gear teeth surfaces above its critical temperature to obtain an austenitic structure throughout its carburized surfaces; a first quenching expedient for cooling the workpiece at a rate greater than the critical cooling rate of its carburized case to a uniform metastable austenitic temperature just above the martensitic transformation temperature; opposed dies, each having an outer peripheral profiled surface, for rolling the gear teeth surfaces to a desired outer peripheral profiled shape while holding the temperature of the workpiece in the uniform metastable austenitic temperature range, the dies being operable such that the gear surfaces first undergo densification by rolling involving substantial compaction of the material in the gear tooth surface layers resulting in a collapse of the pores initially existing near the gear tooth surface region, then plastic deformation as a result of the rolling and sliding movements in the metastable austenitic temperature range with resultant strengthening of the gear teeth; and a second quenching expedient for cooling the workpiece through the martensitic range for hardening the carburized gear teeth surfaces.
The method and apparatus described by United States Patent Application Publication No. 2004/0219051 are limited to working only the outside diameter of a gear by a rolling gear die. The technique of United Stated Patent Application Publication No. 2004/0219051 A1 is not applicable to working the inside diameter of such a gear or to making parts other than those that can be made by rolling operations. It would, of course, be desirable to make high strength parts in addition to gears and to have the ability to work the inside diameter as well as the outside diameter of such a part.